Organic light emitting diodes (OLEDs) are particularly useful for lighting because they can relatively easily and cheaply be fabricated to cover a large area on a variety of substrates. They are also bright and may be colored or white (red, green and blue) as desired. In this specification references to organic LEDs include organometallic LEDs, and OLEDs fabricated using either polymers or small molecules. Examples of polymer-based OLEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
To aid in understanding embodiments of the invention it is helpful to describe an example structure of an OLED lighting tile. Thus referring to FIG. 1a, this shows a vertical cross-section through a portion of an OLED lighting tile 10 comprising a glass substrate 12 on which metal, for example copper tracks 14 are deposited to provide a first electrode connection, in the illustrated example an anode connection. A hole injection layer 16 is deposited over the anode electrode tracking, for example a conductive transparent polymer such as PEDOT: PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene). This is followed by a light emitting polymer (LEP) stack 18, for example comprising a PPV (poly(p-phenylenevinylene)-based material: The hole injection layer helps to match the hole energy levels of this layer to the anode metal. This is followed by a cathode layer 20, for example comprising a low work function metal such as calcium or barium with an optional electron injection layer (not shown) such as lithium fluoride for energy matching, over which is deposited a reflective back (cathode) electrode 22, for example of aluminum or silver.
The example of FIG. 1a is a “bottom emitter” device in which light is emitted through the transparent glass or plastic substrate. However a “top emitter” device may also be fabricated in which an upper electrode of the device is substantially transparent, for example fabricated from indium tin oxide (ITO) or a thin layer of cathode metal (say less than 100 μm thickness). Referring now to FIG. 1b this shows a view of the light emitting tile 10 of FIG. 1a looking towards the LEP stack through the substrate 12, that is looking into the light-emitting face of the device through the “bottom” of the device. This view shows that the anode electrode tracks 14 are, in this example, configured as a hexagonal grid or mesh, in order to avoid obscuring too much light from the LEP stack. The (anode) electrode tracks are connected to a solid copper busbar 30 which runs substantially all the way around the perimeter of the device, optionally with one or more openings, which may be bridged by an electrical conductor) to facilitate that connection to the cathode layer of the device.
It is generally desirable to reduce the amount of light obscured by the tracking, more precisely to improve the ratio of the voltage drop across a tile to the epiture ratio which is lost to the tracking. The inventor determined the resistance of the example mesh of FIG. 1b in different directions in the plane of the device and established that the sheet resistance is substantially the same in any direction, and for various different types of mesh, triangular, hexagonal, square and so forth.
Known OLED electrode structures are described in WO2009/127175 and US2004/0263085.